The invention relates to a stereoscopic display apparatus for displaying a solid image from a plurality of 2-dimensional images at different visual point positions and, more particularly, to a stereoscopic display apparatus for performing a stereoscopic display which has been known as an image type holographic stereogram.
Hitherto, various kinds of studies and developments have been executed with respect to a display method which can stereoscopically observe an image. The conventional stereoscopic display method relates to the double-eye type represented by a glasses type and is a system in which different images are seen to the right and left eyes and a stereoscopic feeling is obtained by a vergence of both eyes or a parallax of both eyes. There is a Lenticular method of the multieye type as an extension of the double-eye type. The principle of stereoscoptic observation is similar to that of the double-eye type. According to such a conventional stereoscopic display method, even when the observer moves the head to the right and left, no difference (kinetic parallax) occurs between the solid images which are observed, so that only an unnatural solid image can be seen. A holographic stereogram can be mentioned as means for eliminating such problems. In the holographic stereogram, a 2-dimensional video image including a parallax is recorded on slit-like segment holograms which are elongated in the vertical direction and a number of such segment holograms are arranged in the horizontal direction. Therefore, even when the observer moves the head to the right and left, a natural stereoscopic feeling can be obtained. There is also a holographic stereogram including a parallax in the vertical direction. For example, when considering a holographic stereogram having a horizontal parallax, first, as shown in FIG. 1, objects 332 and 334 are photographed while changing the camera position from a position 330-1 to a position 330-n. As shown in FIG. 2, a laser beam 342 is subsequently irradiated to a film 336 obtained by the photographing in FIG. 1. A transmission light which passed through the film 336 is projected onto a diffusing plate 340 through a lens 338, thereby generating an object light 344. A slit 350 of a slit plate 348 is located in front of a hologram dry plate 352 in correspondence to the photographing position. An interference fringe (phase distribution) is obtained by an interference between a reference light 354 and the object light 344 and is exposed and recorded onto the hologram dry plate 352.
Further, as shown in FIG. 3, there is also a method of forming a hologram which was imaged. That is, a laser beam (reproduction light) is irradiated onto the hologram dry plate 352 formed in FIG. 2 so as to be converged by a reproduction light source 355 which is shown as a virtual image. Another hologram dry plate is provided at the display position of the image due to the object light 344 which was wave front converted by the exposed hologram dry plate 352. The hologram dry plate is exposed by irradiating the reference light 362 thereto, thereby forming an imaged holographic stereogram (hereinafter, referred to as an "image type holographic stereogram") 360. As shown in FIG. 4, a reproduction light 364 is irradiated to the image type holographic stereogram 360, the reproduction light is converted into the wave front, so that a solid image to be seen from a visual field region 366.
In case of performing a stereoscopic display, it is desirable that a solid image exists near the hologram surface in order to reduce a fatigue of the eyes of the observer. In the holographic stereogram of FIG. 1, it is necessary to convert the image photographed by the camera in order to reconstruct so as to overlap a solid image onto the hologram surface. On the other hand, in an image type holographic stereogram of FIG. 4, since a 2-dimensional image exists on the hologram surface, the solid image can be overlapped onto the hologram surface without converting the image. There is also an advantage such that even when a wavelength of reference light which is irradiated to the hologram changes, the image plane exists on the hologram surface and no color aberration occurs and the like. Therefore, it can be said that a solid image can be more easily seen in case of the stereoscopic display by the image type holographic stereogram.
Such a hologram, on the other hand, can be electronically displayed by using, for example, a space light modulating device using a liquid crystal. To electronically display a hologram by using the space light modulating device, however, it is generally necessary to obtain a phase distribution of the light from a 3-dimensional object to be displayed by a calculation by a computer. With respect to the image type holographic stereogram, it is necessary to calculate a phase distribution from a 2-dimensional image. To calculate the phase distribution, the hologram surface is divided into micro hologram regions. A phase distribution is calculated from the position coordinates and luminance of all of the sampling points of the object with regard to one microregion. Such a calculation is executed with respect to all of the micro hologram regions. Consequently, a calculation amount is extremely large even in case of the image type holographic stereogram to perform the phase calculations for the 2-dimensional image as a target. Each time the content of the 2-dimensional image to be displayed changes, it is necessary to calculate the phase distribution. A load of the calculations by the computer is large. It is demanded to improve such a point.